Geologic Time

Geoscientists are a unique group of scientists for several reasons, but mostly because we work with modern environments as well as interpret ancient environments in the rock record. Therefore, it is of the utmost importance that we as scientists understand how old the rocks are that we are working with, so that we can calculate rates, ages, and determine when geologic events happened. But how do we talk about time, and how do we know how old our rock formations are?

Geologic Time Scale

gsa_tsThe geologic timescale is the most common way geologists organize and communicate major periods of the Earth’s past. The timescale presented at left shows the four major eras (Precambrian, Paleozoic, Mesozoic, Cenozoic), with the oldest on the right and youngest at the top left.  The eras are broken down into periods, which represent smaller units of time. The International Commission on Stratigraphy revises the timescale annually. These updated versions are available in multiple languages and are free to download:

International Chronostratigraphic Chart

Dating the Rocks

Everyone knows geologists love rocks, but when we talk about dating them, we’re not talking about going to a fancy restaurant and ordering a nice pasta dish with our favorite chunk of granite. Dating refers to several methods we use to measure how old a rock is. There are two main ways to determine the age of rocks: relative and absolute dating.

Relative Dating

adri_cephalopod
Adriane holding a rock that contains a fossil of a cephalopod- a marine animal that was related to squids and octopus, but has a shell on the outside of its body.

The most common and oldest method is to use fossils to tell the relative age of rocks. This means we can determine the relative order of geologic events that happened through time and whether one rock formation is older than another. Fossils are contained within sedimentary rocks, which are rocks that are formed over time from the accumulation of sediment, such as in a lake, ocean basin, or river. When those sediments become buried over time and eventually re-exposed, either in outcrops (exposed rocks) or road cuts, we can examine the rocks for its fossil assemblage.

Through several decades of intense study, paleontologists know which fossils are older than others, so by comparing the fossils in one rock outcrop to another, we can relatively tell which one was deposited first, or which is older.

Learn more about the geologic principles that help us in relative age dating of rock units.

Absolute Dating

jen_strataa
Jen standing on a beach with a huge outcrop in the background. Notice how the rocks are layered, with those at the bottom light colored, and those overlying darker. Each layer at this outcrop contains different types of fossils.

Until about the middle of the last century, determining if a rock was younger or older compared to another was the best geologists could do in terms of age dating. The development of the mass spectrometer in the 1950’s revolutionized dating rocks. Mass spectrometers are able to precisely measure the ratios of certain isotopes (two or more forms of the same element with the same number of protons, but which differs in the amount of neutrons) of elements  to get the absolute (actual numerical) age of a rock. Absolute age dating usually involves measuring the radioactive decay of isotopes of minerals contained within a rock. For example, some of the oldest rocks on Earth are located in the Pilbara Craton of western Australia. These rocks contain the mineral zircon, which contains both the elements thorium (Th) and uranium (U) in its crystal structure. Over very long periods of time, uranium decays to lead. Thus, by measuring the ratio of lead to uranium within a zircon mineral, we can determine the age of the zircon mineral with very low error (or high precision). The half-life of uranium with 238 neutrons is about 4.47 billion years, and uranium with 235 neutrons has a half-life of 704 million years. Therefore, due to the very long half-life of uranium 238, we can use that element and its decay to lead to date the age of the Earth. Other elements have different half-lives and may be more suitable for dating younger rocks. For example, the half-life of potassium (K40), which decays to argon (Ar40) is about 1.3 billion years.

A Word about C14 Dating

One of the most popular methods in absolute age dating is the use of carbon 14 (carbon atoms with 14 neutrons in the nucleus). Because of its wide use in archaeological studies and for historical objects, there is some misunderstanding that C14 dating is used very commonly in geology as well. Although this dating technique is not unheard of to date very young sediments, it is not commonly employed in most geologic studies. The half-life of C14 is only 5,730 years, meaning it is only useful on sediments younger than about 50,000 years.

Combining Relative & Absolute Age

Combining absolute age with relative age of rock formations can lead to really strong data to interpret! As previously mentioned, fossils tell us the relative series of events that occurred, or which formations of rock are older than another. Absolute age tells us the actual age of a formation, but is often harder to obtain than relative age due to the care that must be taken to obtain the minerals needed, as well as the lab time and expense it takes to process samples. In addition, not all rocks contain suitable minerals for absolute age dating, especially sedimentary rocks. However, we do have ages for some sedimentary rocks due to the presence of bentonites, or ancient volcanic ash horizons, that do contain minerals that can be used for absolute age dating. Fossils aside, there are several other methods to determine the ages of rocks.

Geomagnetic Polarity Timescale

ck95
The Geomagnetic Polarity Timescale of Cande and Kent, 1985. Black bars are normal times, and white bars are times of magnetic reversals. Numbers at the boundary between reversals on the left is age in millions of years; numbers and letters on the right are called chrons.

One of the most common ways to determine the age of rocks and sediments of Cenozoic (0-66 million years ago) and Mesozoic (66-252 million years ago) age is to measure the Earth’s ancient polarity to determine age. Polarity is the location of the north and south poles, which today are located in the Arctic and Antarctica, respectively.

However, there were other times in the Earth’s past when polarity was flipped, so that the north pole was in the Southern Hemisphere and the south pole was in the Northern Hemisphere! This simply means the magnetic pole is swapped, the landmasses stay stationary during this process. This change in polarity is measured from sedimentary rocks and in ocean sediments that are drilled from the sea floor using a special instrument. When sediment settled in a lake, ocean basin, or river, magnetic minerals aligned themselves to the north and south poles. When the sediments became buried and became rocks, they retained the magnetic signal at the time they were deposited. Thus, the ancient polarity of the Earth is preserved in sedimentary rocks.

When polarity was ‘normal’, the north pole is in the Northern Hemisphere and the south pole in the Southern Hemisphere, like today. At times of flipped or reversed polarity, the opposite was true. Thus, the geomagnetic polarity timescale records normal polarity times as black bars, and reversed polarity times as white bars. The figure at left is an example of a Geomagnetic Polarity Timescale, with the white boxes representing times of reversed polarity, and black boxes indicating normal polarity. In the polarity timescale presented here, some of the ages between reversals were calibrated using absolute age dating techniques when volcanic ash horizons were available. When not available, ages are determined using biostratigraphy.

Biostratigraphy

Biostratigraphy is the use of fossils to date rocks and sediments, and has been a technique used by geologists since the early 1800 and still very common today. Biostratigraphy relies on the ranges of fossil organisms, specifically when an organism first evolved, or its first appearance, and when an organism went extinct, known as its last appearance. From these first and last appearances, fossil zones and subzones were created. But not all fossils are useful in biostratigraphy: there are certain criteria an organism must meet to be used as an index fossil, or a fossil that is useful in relative age dating. These criteria state that an index fossil must have been geographically widespread, be easy to identify, common in the rock record, and the organism must have evolved rapidly.

But what organisms are useful in biostratigraphy? There are several, and the useful fossil groups change through geologic time as species went extinct and new ones evolved. In rocks of  Paleozoic age (541-251 million years ago), useful organisms to tell time include conodonts, chitinozoans, graptolites, and trilobites. In rocks of Mesozoic age (251-66 million years ago), the most useful organisms for biostratigraphic use include foraminifera, ammonites, fossil pollen, and nannofossils. In sediments and rocks of Cenozoic age, foraminifera, fossil pollen, and nannofossils are the most useful biostratigraphic organisms.

orgs-for-website
Common fossils used for biostratigraphy. Note that images are not to scale. Conodonts, chitinozoans, foraminifera, fossil pollen, and nannofossils are all measured on a milimeter scale, whereas graptolites, trilobites, and ammonites reach anywhere from 1 cm to tens of centimeters in length.

Astrochronology

Perhaps the most precise and also newest ways to determine the age of rock on the Geologic Timescale is through astrochronology. Astrochronology relies on the three orbital parameters of the Earth to ‘tune’ our geologic record. Let us first introduce these orbital parameters, or Milankovitch Cycles, which were first discovered by the Serbian astrophysicist Milutin Milankovitch.

Milankovitch cycles describe how the Earth’s orbit around the sun changes through time. Each of the three orbital cycles operate on separate timescales:

  1. Eccentricity is the shape of Earth’s orbit as it moves around the sun. This path changes from being elliptical (or oval-shaped) to nearly (but never perfectly) circular. The change from ovate to circular takes place every 100,000 years.
  2. Obliquity is the change in the axial tilt of the Earth. Currently, the tilt of the Earth is 23.5 degrees, but this varies between 22.1 to 24.5 degrees. This change in axial tilt takes about 41,000 years.
  3. Precession is the change in the ‘wobble’ of Earth about its axis. Think of a spinning top wobbling about its axis as it begins to slow down; this is how precession operates. One precession cycle take about 23,000 years.

For a visualization of each of the orbital parameters, check out the University of Wisconsin Madison’s website here.

milankovitch-cycles
The three orbital parameters: eccentricity, obliquity, and precession.

The three orbital parameters substantially influence climate on geologic timescales. For instance, when the Earth’s obliquity increases (shifts more towards 24.5 degrees rather than 22.1 degrees), this leads to more exaggerated seasons- warmer summers and colder winters. When the obliquity (angle of tilt) decreases, this leads to milder winters and summers. One hypothesis about how continental ice sheets grow is: during periods of low obliquity, snow forms in the winter but can’t be melted entirely because of the cool summer. That way the snow keeps accumulating, building up into a big ice sheet. For a more extensive and in-depth discussion of Milankovitch cycles and theory, visit the NASA Earth Observatory site here.

So what do orbital parameters have to do with constructing the geologic record? The simple answer is: a lot! Because these cycles influence climate and climate influences the types of rocks that are deposited, orbital cycles can be preserved in the rock record by cyclic bedding. One example is packages of rocks, such as limestones and shales, that are deposited due to environmental changes such as sea level. Although cyclic sediments themselves are very informative, they alone cannot help us refine our geologic timescale and reduce the error in ages between periods and epochs. As stated above, the best way to obtain the absolute age of a rock formation is through measuring the radioactive decay of certain isotopes in a mineral. So, because we know the period at which Milankovitch Cycles operate, and where in each cycle the Earth is today, we can therefore extrapolate the cycles back in time. Recall that we know the exact time it takes for one cycle of eccentricity, obliquity, and precession to occur, so we can use those ages to determine the age of different periods and epochs.

anchored_rock
A model of how geologists use astrochronology to date the rock record. A) An outcrop of limestone and shale layers. The layers exhibit cyclic bedding, indicating they reflect Milankovitch cycles, but we’re not sure the age of the rock, so it’s not time informative. B) We know the eccentricity cycle, and that one cycle occurs every 100,000 (100 ky) years. Again, we’re not sure how the eccentricity cycle fits to the limestone and shale beds. C) By using radiometric age dating on one of the shale beds, we obtain an age of 250.562 million years (Ma). D) Because we know the periodicity of eccentricity cycles, we know where that age falls on the eccentricity cycle. The rock record is now ‘anchored’ in time by the radiometric age (‘tie point’). E) Now that we know the age of one shale layer, and where that falls on the eccentricity cycle, we can calculate the ages of the other beds in the formation by adding 100,00 years to every cycle on the eccentricity scale. F) Thus, we can now use the eccentricity cycle to date this rock formation!

Another way to think about cyclic sediments and how they can be of use to calibrating geologic time is by imaging the rock record, such as the cyclic limestone and shale beds figured here, as a ‘floating timescale’. We know the rocks were deposited in response to different climate conditions controlled by Milankovitch cycles, but we don’t know the age. Through the use of one or more radiomentric ages obtained from the rock formation, we can use these ages as ‘tie points’ in time. Thus, we can then anchor the rocks in time, and from there, extrapolate what the ages of the rocks above and below must be.

This method has greatly cut down the error on previous estimates of geologic time, and is now the preferred method used by geoscientists to determine the age at which a geologic event occurred.

The Big Picture

We’ve just discussed how our geologic timescale is created and how geoscientists determine the age of the rocks. But why is this so important, and what’s the big deal? To geologists, age is everything. Without it, we would not be able to make interpretations from the rock record regarding things such as rates of evolution or how fast climate changed. In addition, knowing the age of different rock formations around the world has allowed for the correlation of rocks within a state, across a continent, or even across oceans!